With advancing adult age there is an alteration in the regulation of glucose metabolism; specifically a deterioration in glucose tolerance. There is little consistent evidence as to the extent and/or mechanisms responsible for these age-related changes in glucose metabolism. The objective of these studies is to characterize in reliable, quantitative terms the basal rates of glucose production, uptake, oxidation, and recycling of glucose carbon in healthy young adult and elderly subjects. In addition, regulation of these parameters of glucose metabolism will be explored during infusions of insulin and/or glucose and the effects of diet on these aspects will be studied. The approach used will be based on a primed-constant infusion of tracer quantities of labeled -(3H and 13C) glucose.